Superluminality of Dynamical Chern-Simons

Classical tests of extensions of general relativity are a well-established avenue for determining the viability of such theories. One such test and extension of GR is Shapiro's time delay and dynamical Chern-Simons gravity. Computing the time delay for a gravitational theory allows us to determine whether or not the corresponding theory is causal, and what the corresponding constraints are. Here, we compute the time delay of dynamical Chern-Simons by deriving the corresponding dispersion relation, and placing it on a gravitational wave background. In doing so, we find superluminal modes that allow us to bound the value of the dCS coupling constant. The implications this has on ultraviolet completions of dCS are discussed.